1. Field of the Invention
The present invention relates to an electric discharge lamp, such as a fluorescent lamp, having an electrode with a filament and lead wires connected thereto.
2. Description of the Related Art
Generally, a hot cathode electric discharge lamp includes a pair of electrodes inside and at each end of a cylindrical glass tube. The electrode of the electric discharge lamp, comprises a filament connected to lead wires which extend through and are supported by a stem formed in an end portion of the glass tube.
In this kind of hot cathode electric discharge lamp, an anomalous discharge may be caused: after an emissive material (emitter) depletion at the end of the lamp's life, by a preheating current increase, or by an electrode material or lead wire material dispersing and adhering on the surface of the stem. When this occurs, the electric discharge lamp will generate heat by a short circuiting of the lead wires and the preheating current will flow on the surface of the stem, creating the possibility of melting the glass around the stem and the glass tube.
To prevent the stem and the glass tube from melting by an anomalous discharge of a hot cathode electric discharge lamp, Japanese Patent Laid-Open No. 11-203934 (hereafter JP '934) discloses a construction of a thermal fuse which interrupts current flow at the time of an anomalous discharge. The JP '934 patent publication discloses the construction of a thermal fuse positioned on an outer peripheral surface of the glass tube near an electrode.
According to JP '934, at the time of an anomalous discharge, temperature will have the tendency to increase faster at a location near the electrode and increase slower at locations further from the electrode in a longitudinal direction of the glass tube. For this reason, there is an issue that variation of temperature detected by the thermal fuse will be significant, depending on the installation position of the thermal fuse.
Nevertheless, in the technology disclosed in JP '934 mentioned above, the location of the thermal fuse with respect to the electrode is ambiguous. JP '934 neither discloses nor suggests the relative position between the thermal fuse and the electrode, nor does it teach the relationship between the diameter of the glass tube and the placement of the thermal fuse. JP '934 only discloses a possibility that the variation in operating temperature of the thermal fuse may be significant depending on the thermal fuse is located.
There is also a need to avoid relatively high temperatures in an electric discharge lamp used as a back light source for a transmissive liquid crystal display (LCD). In a transmissive LCD, variation in the operating temperature of a thermal fuse may adversely affect components used to manufacture an LCD well before and even if the glass and stem area do not reach the melting point.